1. Field of the Invention
This invention relates to noncontact multiple sheet detecting apparatus that utilizes ultrasonic techniques.
2. Description of the Prior Art
In many different kinds of devices such as printing presses and copy reproducing apparatus, sheets are serially fed through different work stations wherein they are operated upon. When multiple sheets are accidentally forwarded, jams may occur at processing stations which result in machine malfunctions. In these apparatus, it is important to detect when superposed sheets are fed. This is especially true with the high speed sequential sheet feeding requirement of apparatus such as some copy duplicators.
Various multiple sheet detecting apparatus have been employed in the past. Many of these employ mechanical sensors to detect the multiple sheet feed condition. See, for example, U.S. Pat. No. 3,396,965. Some mechanical systems employ roller members which sense paper thickness. With mechanical systems where moving parts are involved, there are, of course, wear problems and also the difficulty of maintaining close mechanical tolerances. Not only is this a problem due to wear, but also errors can occur when sheets of material of nonuniform thickness are fed, or if a sheet is momentarily bowed when passing by a mechanical multiple sheet sensor.
Noncontact multiple sheet detecting apparatus provide a number of advantages over the mechanical contact-type systems. In one such detector disclosed in Canadian Pat. No. 929,632, a multiple sheet feed condition is determined by measuring the transmittance of light of the sheet(s) being fed and computing the difference in transmittance of such sheet(s) with respect to a reference transmittance provided relative to a sample sheet. Although such an apparatus would be effective, a problem exists in that if the transmittance of the sheet(s) being fed should change, then, the reference would have to be changed. Moreover, if the light sources should differently vary in intensity of illumination, then errors can possibly develop. Another similar electro-optical system is disclosed in U.S. Pat. No. 3,614,419 wherein the transmittance of successively fed sheets are compared to determine if there has been a double sheet condition.
Sheet detectors often determine the average transmittance of a sheet(s) to determine a multiple feed condition. However, if certain sheets that are fed have large dark pictures formed thereon or are covered with printed material while others are plain paper, it is difficult to utilize average transmittance to detect multiple sheet feeds. Moreover, electro-optical systems often require homogeneous optical density in the fed sheets; and reliable operation of such a system can be effected by color or printing on the sheets. Further, a clean optical path is often a necessity; thereby requiring further means for insuring a dirt, dust-free environment.
It should be noted that prior multiple sheet detectors of both the contacting and noncontacting variety are ineffective if intermixed sheets of paper of various thicknesses are to be fed along a path.